The invention relates to xerographic process control and, more particularly, to the improvement for measurement and adjustment of tone reproduction curves by using real-time control of the tone reproduction curve by redefinition of look tables from fitting of in-line enhanced toner area coverage sensor data.
In copying or printing systems such as a xerographic copier, laser printer or inkjet printer, a common technique for monitoring the quality of prints is to artificially create a test patch of a predetermined desired density. The actual density of the printing material, toner or ink for example, in the test patch can then be optically measured to determine the effectiveness of the printing process in placing this printing material on the print sheet.
In the case of xerographic devices such as a laser printer, the surface that is typically of most interest in determining the density of printing material thereon is the charge retentive surface or photoreceptor on which the electrostatic latent image is formed and subsequently developed by causing toner particles to adhere to areas thereof that are charged in a particular way. In such a case, an optical device, often referred to as a densitometer, for determining the density of toner on the test patch is disposed along the path of the photoreceptor directly downstream of the development unit. There is typically a process within the operating system of the printer to periodically create test patches of the desired density at predetermined locations on the photoreceptor by deliberately causing the exposure system thereof to change or discharge as necessary the surface at the location to a predetermined extent.
The test patch is then moved past the developer unit and the toner particles within the developer unit are caused to adhere to the test patch electrostatically. The denser the toner on the test patch, the darker the test patch will appear in optical testing. The developed test patch is moved past a densitometer disposed along the path of the photoreceptor and the light absorption of the test patch is tested. The density of toner on the patch varies in direct relationship to the percentage of light absorbed by the test patch.
Xerographic test patches that are used to measure the deposition of toner on paper to measure and control the tone reproduction curve (TRC) are traditionally printed on inter-document zones of the photoreceptor belts or drums. Generally, each patch is a small square that is printed as a uniform solid halftone or background area. This practice enables the sensor to read values on the TRC for each test patch.
Many xerographic printing system process controls move physical actuators such as developer bias, charge level and raster output scanner (ROS) intensity to maintain the TRC as measured by an in-line enhanced toner area coverage (ETAC) sensor. The controls maintain the TRC at 3 points, however, there is still some variation at the control points due to dead band control, and there is still some variation between the control points due to changes in the shape of the TRC. There are insufficient actuators and insufficient latitude to control the entire TRC to the desired accuracy. This variation causes objectionable changes, especially in overlay colors which are printed using more than one of the printer primary colors.
Accordingly, because of the difficulty in monitoring and controlling the toner development process, various approaches have been hereinbefore devised.
U.S. Pat. No. 5,963,244 to Mestha et al. discloses the idea of sensing the TRC at discrete intervals and doing a least squares fit to project an entire TRC. The tone reproduction curve is recreated by providing a look-up table for reconstruction of the TRC. The look-up table incorporates a co-variance matrix of elements containing end-tone reproduction samples. The matrix multiplier responds to sensed developed patch samples and to the look-up table to reproduce a complete tone reproduction curve. A control reacts to the reproduced tone reproduction curve to adjust machine quality.
U.S. Pat. No. 5,749,020 to Mestha et al. discloses the idea of describing TRC variations using a set of orthogonal basis functions. The basis functions are derived by decomposing sample tone reproduction curves to provide a predicted tone reproduction curve. The predicted tone reproduction curve is melded with a discrete number of tone reproduction samples to produce a reconstructed TRC for machine control.
U.S. Pat. No. 6,035,152 to Craig et al. discloses a method for measurement of tone reproduction curves. A setup calibration TRC is generated based on preset representative halftone patches. A test pattern comprising a plurality of halftone patches is marked in the inter-document zone of the imaging surface. A relative reflection of each of the halftone patches is entered into a matrix and the matrix is correlated to a plurality of print quality actuators. A representative TRC is generated based on the matrix results. A feedback signal is produced by comparing the representative TRC to the setup calibration tone curve and each of the print quality actuators is adjusted independently to adjust printing machine operation for print quality correction.
U.S. Patent No. 5,777,656 to Henderson describes the concept of using lookup tables to adjust a measured TRC to match a target TRC. The method of maintaining tone reproduction for printing comprises the steps of marking representative halftone targets on an imageable surface with toner sensing an amount of toner on each of the representative halftone targets, generating a representative TRC based on the sensed amount of toner on the representative halftone targets, producing a feedback signal generated by comparing a representative TRC to a setup calibration tone curve and adjusting pixel data of each pixel of the final halftone image to compensate for deviation between the representative TRC and the setup calibration tone curve.
U.S. Pat. No. 5,649,073 to Knox et al. discloses a method and apparatus for calibrating gray reproduction schemes for use in a printer. The calibration system includes a test pattern stored in a memory and providing a plurality of samples of combinations of printed spots printable on a media by the printer. A gray measuring device is included to derive a gray measurement of the samples of printed spots. A calibration processor correlates the gray measurements with a combination of spots having a particular spatial relationship and derives parameters describing the printer response to the combination. The calibration processor generates from the derived parameters at least one non-linear gray image correction function then stores the generated gray image function calibration in a calibration memory. A means is provided to apply the gray image correction stored in the calibration memory to calibrate a printer using a halftone pattern.
U.S. Pat. No. 5,612,902 issued to Stokes discloses a method and system for automatic characterization of a color printer. A relatively few number of test samples are printed and measured to create an analytic model which characterizes a printer. The analytical model is used in turn to generate a multi-dimensional look-up table that can then be used at one time to compensate image input and create a desired visual characteristic in the printed image.
Accordingly, it is an object of the present invention to provide a new and improved method for process control by providing real-time adjustment to a target TRC by means of real-time update of machine look-up tables.
A method and system are provided for real-time control of tone reproduction curves. The machine comprises a moving photoreceptor, a means for storing a target tone reproduction curve and a means for updating a current tone reproduction curve look-up table (LUT). The means for updating a current tone reproduction curve LUT comprises a means for scheduling the depositing and measuring of the test patches, a means for depositing halftone test patches on the photoreceptor, a means for measuring the density of the halftone test patches and generating a measured tone reproduction curve, a means for computing differences between the measured tone reproduction curve and the target tone reproduction curve and fitting the differences to a three parameter sine function, a means for calculating a new tone reproduction curve LUT based on the target tone reproduction curve and the fitted differences, a means for limiting differences between the new tone reproduction curve LUT and a current tone reproduction curve LUT to a predetermined maximum magnitude, and a means for loading the current tone reproduction curve LUT with the new tone reproduction curve LUT.